Cellular compartmentation in arginine metabolism of Neurospora will be studied in several ways: 1. The enzyme carbamyl phosphate synthetase A is an arginine biosynthetic enzyme located in mitochondria. We will study the localization, aggregation, and regulation of its two polypeptide subunits, and the genetic control of these processes. 2. Basic amino acids, including arginine and ornithine, are located in the vacuole of Neurospora. We shall study the mechanism of transport and retention of basic amino acids by the vacuole. 3. We shall study the metabolism of ornithine as it is used by the ornithine decarboxylase and ornithine transcarbamylase reactions. Mutations which affect the transport of ornithine through the mitochondrial membrane will be given special scrutiny. The information above should be almost directly applicable to mammalian cells which have a urea cycle, because similar compartmental features prevail in this pathway in mammals.